Skip a sporadically failing libcxx test

[StephanTLavavej]

Followup to #4263. In the zillions of times that I ran libcxx locally, and in GitHub PR checks, and for MSVC locally, and in MSVC-PR checks, everything was fine - then the very first post-merge GitHub CI encountered a sporadic failure in an inherently bogus detached-threads test. This needs to be SKIPPED because we don't know if it'll pass or fail.

Additionally, I'm recording llvm/llvm-project#75611 that @frederick-vs-ja reported, as mentioned in #4263 (comment).

[frederick-vs-ja]

Some further analyses:

This is probably a bug of MSVC STL and I've opened #4268. Not sure whether we can fix it before vNext.

STL/tests/libcxx/expected_results.txt

Lines 1103 to 1107 in e077eb6

	# Not analyzed. Assertion failed: _CrtIsValidHeapPointer(block)
	std/input.output/string.streams/stringbuf/stringbuf.members/str.pass.cpp FAIL
	std/input.output/string.streams/stringbuf/stringbuf.members/view.pass.cpp FAIL
	std/input.output/syncstream/syncbuf/syncstream.syncbuf.cons/dtor.pass.cpp FAIL
	std/input.output/syncstream/syncbuf/syncstream.syncbuf.members/emit.pass.cpp FAIL

Notes to myself, no change requested: it seems that MSVC will be happy if char(N) is changed to static_cast<char>(N), for an unknown reason.

STL/tests/libcxx/expected_results.txt

Lines 826 to 828 in e077eb6

	# Not analyzed. MSVC emits "warning C4310: cast truncates constant value" for char(0xc3).
	std/input.output/filesystems/class.path/path.member/path.charconv.pass.cpp:0 FAIL
	std/input.output/filesystems/class.path/path.member/path.charconv.pass.cpp:1 FAIL